Coke is an indispensable ingredient of many industrial and metallurgical processes, for example, blast-furnace operations, certain smelting operations, certain chemical processes and other purification processes. In order for such processes to be refinable and economical, it is very desirable that the coke should be relatively inexpensive, of uniform predictable quality, and in a suitable prepared form.
Additionally, the need for a smokeless fuel, such as coke, which would not obnoxiously pollute controlled environments especially in the face of fluctuating oil prices, has largely been responsible for the emphasis on the development of sophisticated coke ovens which are able to economically produce high quality coke of predictable coke-characteristics such as porosity, volatile content, coherence, swelling, coke-reactivity, mechanical strength and combustibility. The aforesaid coke-characteristics are influenced in part by the control of operating conditions per se within the coke oven; also however, the coal grade as well as the nature and the extent of preparation of the coal play an equally important role.
Coke ovens are of many types; in the basic or fundamental form, a coke oven is known to be built in the form of a firebrick-chamber in a substantially hemispherical shape. Coke ovens of such type are termed "beehive coke ovens". Invariably nowadays, with minor exception, beehive coke ovens are considered old fashioned and, being somewhat wasteful, are obsolete; instead, by-product ovens are used now for many applications.
As a first step in coke-production, coal is prepared and charged into the top of empty coke ovens and levelled to a uniform layer. Almost immediately after charging, because of the heat retained in the oven from previous charges, or because of preheating, gases start evolving from the charged coal. The evolved gases start burning because of combustion-air admitted in controlled quantities. In some ovens there is a provision to collect effluent gases whereby a portion of the heat contained in the effluent gases is utilized for generating steam, through the use of waste heat boiler.
The efficiency of operation of a coke oven is usually measured by the production of good and uniform quality coke, minimum consumption of fuel gas, minimum loss of volatile products by leakage; also contributing to the efficient operation of a coke oven is the efficient performance of the heating flues within the coke oven, which performance is influenced by the heat conductivity characteristics of the coal charge; for certain vertical flue ovens, gas and preheated air are admitted at the base of the flues causing upwardly directed combustion therein in order to provide additional heat where needed. Any suitable kind of gaseous fuel can be used for combustion in a coke oven, including the coke oven gas itself, or blast furnace gas. Even-heating and proper temperature control are very crucial, and the degree of compactness of the loaded coal, as well as the size of the loaded coal particles and the preloading preparation thereof play a critical role in the production of good quality coke. The degree of compactness not only influences the bulk of the coal which can be charged into the coke oven and but also directly affects the heat transfer patterns within the coke oven.
For any given grade or category of coal, in the interest of economy, it is desirable to use coal portions of normal size which may be in the form of larger chunks and also utilize a wide range of sizes in particulate form, including fine grain portions in the coke ovens.
Loading of fine grain portions per se, of coal or inclusion of fine grain coal in the charge mixture to the coke oven, is found to result in a decrease in the bulk density of the charge mixture on the whole. A reduction of the bulk density has a threefold effect on the performance of the coke oven leaving a lot to be desired.
First and the most obvious, a lower bulk density of the charge results in a reduced capacity of the coke oven with the consequence that in a given time duration, the coke output becomes diminished compared with the production with a normal bulk-density charge.
Second and no less important, the coking time is relatively increased with low bulk density charges, because of poorer heat transfer in the coking mixture. Increased coking time obviously results in poor economy and high cost of production.
A third consequence cannot be ruled out that with low bulk density charges, caused because of fine grain portions of coal, often the useful amount of coke in the output is relatively reduced.